There has been known a method of producing a non-ageing cold rolled steel sheet for deep drawing by the use of a low carbon aluminium killed steel. In this method, a high r value is obtained in the resulting cold rolled steel sheet by the action of AlN precipitated during the heating in a box annealing, and at the same time N is precipitated and fixed by Al, and C is precipitated and fixed in the form of Fe.sub.3 C to give a non-ageing property to the resulting steel sheet. As another method of producing a cold rolled steel sheet with a good ageing resistance, there has been known a method wherein decarburization and denitrogenization are carried out by an open coil annealing.
Both the above described methods are carried out by a batch system, and therefore these methods are inferior to continuous annealing method in the productivity and are poor in the homogeneity of the annealed steel sheet. Moreover, in these methods, a long period of time of heat treatment is carried out, and therefore temper color is apt to develop on the surface of steel sheet due to the enrichment of Si, Mn and the like. Further, when decarburization or denitrogenization is carried out, the decarburized or denitrogenized steel sheet shows the cold-work embrittleness due to the segregation of P in the grain boundary during the slow cooling.
While, the continuous annealing method is free from the drawbacks of the above described batch annealing method. However, in the continuous annealing method, a cycle consisting of a rapid heating, a short time soaking and a rapid cooling is carried out, and therefore as far as a low carbon steel is used, the continuous annealing method cannot develop fully crystal grains and is inferior to the batch method in the ductility and r value of the resulting steel sheet, and is more difficult than the batch method in the fixing of C and N and in the production of non-ageing steel sheet.
In order to obviate the above described drawbacks of the continuous annealing method, various methods for producing a cold rolled steel sheet having a satisfactory property from an extra-low carbon aluminium killed steel even by a continuous annealing cycle have been disclosed. Japanese Patent Application Publication No. 17,490/76, Japanese Patent Laid-Open Application No. 58,333/80 and the like disclose these methods. However, these methods still have the following drawbacks.
(A) It is difficult to produce a steel sheet having substantially non-ageing property unless an extra-low carbon aluminium killed steel having a C content of not higher than 20 ppm is used.
(B) Even in the use of a steel having a C content of as low as not higher than 20 ppm, the resulting steel sheet still has a large planar anisotropy in the r value, elongation and the like, and has a problem for practical use.
While, as a method for producing a steel sheet having excellent deep drawability and ageing resistance and further having small anisotropy, there have been known methods, wherein C and N contained in the steel are precipitated and fixed by using powerful elements for forming carbide or nitride, such as Ti, Nb and the like. The use of Ti is disclosed in Japanese Patent Application Publication No. 12,348/67, and the use of Nb is disclosed in Japanese Patent Application Publication No. 35,002/78. However, in these methods, when a steel contains a large amount of C, the resulting steel sheet is poor in the ductility due to a large amount of precipitates; and reversely when a steel contains a small amount of not more than 50 ppm of C, the C cannot be fully precipitated and fixed unless Ti or the like is used in an amount considerably larger than the stoichiometrically necessary amount for fixing the C. Therefore, unbonded excess Ti and the like also deteriorate the ductility of the resulting steel sheet, and affect adversely the formability of the steel sheet.
Further, Japanese Patent Laid-Open Application No. 81,913/75 discloses a method of securing excellent property in the resulting steel sheet, wherein a very small amount of at least one of B, Nb, Zr, V and Ti is added to a low-carbon aluminium killed steel having a C content of 0.05-0.07%, the steel is formed into a steel sheet, the steel sheet is subjected to a recrystallization annealing, and the annealed steel sheet is subjected to an overageing treatment at a temperature not lower than 300.degree. C. to precipitate the major part of C contained in the steel. However, this method treats always a low carbon steel, and an overageing treatment must be carried out in the continuous annealing.
Further, the inventors have already disclosed a cold rolled steel having ultra-deep drawability, which consists of an extra-low carbon aluminium killed steel having a C content of 0.004-0.006% and an Nb content of 0.026-0.043%, and a method of producing the steel sheet in Japanese Patent Laid-Open Application No. 169,752/81; and further disclosed a high tensile strength steel sheet having ultra-deep drawability, which consists of an extra-low carbon aluminium killed steel having a C content of 0.005-0.009%, an Nb content of 0.027-0.043% and a P content of 0.062-0.082%, and a method of producing the steel sheet in Japanese Patent Laid-Open Application No. 139,654/81. However, the present invention is different from these Japanese laid-open applications in the following two points of (a) C.ltoreq.0.004% and (b) Nb and other elements .ltoreq.0.01%.
The object of the present invention is to solve the above described drawbacks of the conventional technics, and to provide a method of producing a cold rolled steel sheet having good ageing resistance and small anisotropy and adapted for deep drawing.